IPv6: addrconf notify when address is unavailable

My recent change in net-next to retain permanent addresses caused regression.
Device refcount would not go to zero when device was unregistered because
left over anycast reference would hold ipv6 dev reference which would hold
device references...

The correct procedure is to call notify chain when address is no longer
available for use.  When interface comes back DAD timer will notify
back that address is available.

Also, link local addresses should be purged when interface is brought
down. The address might be changed.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>